1. Field of the Invention
The present invention relates to filters, and in particular to low-pass filters with attenuation poles.
2. Description of the Related Art
While most of the literature on passive resistance-capacitance (RC) band-reject, frequency rejecting network (FRN), or notch filters has been dedicated to the bridged full-T filter, another passive RC filter overcomes some of the impracticalities of the bridged full-T filter. That filter is the isolated-integrator band-reject filter, as shown in FIG. 1.
The isolated-integrator band-reject filter has several unique qualities, which the bridged full-T filter lacks. In terms of structure, the isolated-integrator band-reject filter uses capacitors of equal value. This enables the manufacturer to use capacitors of a single production batch, and to grade only one type of capacitor versus many types. Additionally, the capacitors used can be relatively inexpensive NPO ceramic types.
The net capacitance of the filter itself acts to aid the construction of a low-pass filter when incorporated into an active filter design. While attenuation with a voltage source and an open circuit load is symmetrical, the natural design tendency is to include finite impedances for both the voltage source and the load. However, in practice it is more likely that the low-pass pass-band will be gain dominant.
Additionally, the frequency of the isolated-integrator band-reject filter may be tuned in both directions by trimming a single variable resistor. Beyond the advantage of use of a single member, the use of a resistor in particular permits laser trimming or abrasive adjustment methods, which can be applied to mass applied screened-on resistor types in the manufacturing processes. The use of a tuning resistor also enables the reduction of noise during the tuning process as the tuning resistor is connected to ground.
An additional feature of the filter is its ability to provide deep notches even at megahertz frequencies with practical values of components. The notches, or poles, remain deep, even when common component tolerance limits are incorporated. However, infinite notch depth may be attained only with exact and ideal values. Lastly, multiple isolated-integrator band-reject filters can be cascaded if necessary to produce multiple notches as part of the system response.
Additional information regarding resistance-capacitance notch filters is given in the paper “Tunable RC Null Networks,” authored by Ralph Glasgal on pages 70-74 of the October 1969 issue of EEE which is expressly incorporated herein by reference.